This application is related to an application of D. Morgan Tench and John T. White entitled xe2x80x9cImproved Semiconductor Wafer Plating Cell Assemblyxe2x80x9d, which is being filed on the same date as this application.
1. Field of the Invention
This invention is concerned with electroplating of semiconductor wafers, and in particular with the formation of copper integrated circuits (IC""s) on semiconductor chips.
2. Related Prior Art
The electronics industry is in the process of transitioning from aluminum to copper as the basic metallization for semiconductor IC""s. The higher electrical conductivity of copper reduces resistive losses and enables the faster switching needed for future generations of advanced devices. Copper also has a higher resistance to electromigration than aluminum.
The leading technology for fabricating copper circuitry on semiconductor chips is the xe2x80x9cDamascenexe2x80x9d process (see, e.g., P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p. 32; U.S. Pat. No. 4,789,648 to Chow et al.; and U.S. Pat. No. 5,209,817 to Ahmad et al.). In this process, vias are etched through and trenches are etched in the chip""s dielectric material, which is typically silicon dioxide, although materials with lower dielectric constants are desirable. A barrier layer, e.g., titanium nitride (TiN) or tantalum nitride (TaN), is first deposited into the trenches and vias by reactive sputtering to prevent Cu migration into the dielectric material and degradation of the device performance. Next, a thin copper seed layer is deposited by sputtering to provide the enhanced conductivity and good nucleation needed for copper electrodeposition. Copper is then electrodeposited into the trenches and vias. Excess copper deposited over the trenches and vias and in other areas is removed by chemical mechanical polishing (CMP). The xe2x80x9cdual Damascenexe2x80x9d process involves deposition in both trenches and vias at the same time. As used in this document, the general term xe2x80x9cDamascenexe2x80x9d also encompasses the dual Damascene process.
Damascene electroplating is generally performed on full silicon wafers, which are disks typically 8 inches (200 mm) in diameter and 0.03 inches (0.75 mm) thick. The industry trend is toward wafers of even larger diameters. Currently available wafer plating equipment employs a cathode assembly that includes a metallic backing plate, an insulating plastic housing, and a special metallic ring that makes electrical contact to the copper seed layer around the perimeter of the xe2x80x9cplatedxe2x80x9d side of the wafer, i.e., the side of the wafer that is electroplated with copper. A concentric gasket (or o-ring) of smaller diameter is used to form a seal between the wafer plated-side surface and the plastic housing so as to prevent intrusion of the plating solution into the contact area and to the non-plated side of the wafer (opposite to the plated side). During plating, the electrolyte is pumped through at least one tubular nozzle directed at the wafer surface to provide bath agitation. The wafer is typically plated in the plated-side-down configuration and the cathode assembly is rotated to enhance the rate and uniformity of solution flow across the wafer surface.
A major problem with current wafer plating systems is that the gasket or o-ring used to form a seal to the wafer requires a reasonably thick mechanical support structure which protrudes past the wafer plated surface, impeding solution flow and causing nonuniform copper deposition. In addition, to accommodate the electrical contact assembly and protective plastic housing, the plating tank is made substantially larger in diameter than the wafer plated area so that the wafer perimeter of the plated area tends to be overplated because of the additional current paths through the additional plating solution. Note that the excess copper layer (above the trenches and vias) must be thin and uniform for practical CMP processing. Complicated baffles and shields are used in conjunction with cathode rotation to improve copper plating uniformity but these increase the complexity and expense of automation and do not provide optimum plating results. In addition, the requirement of cathode rotation is more easily fulfilled by exposing the wafer to the solution in the plated-side-down configuration for which trapping of bubbles within fine trenches and vias is a problem.
There is a critical need for a low-profile wafer plating assembly offering minimum impediment to solution flow across the wafer surface. Such an assembly would be valuable in providing more uniform Damascene copper plating and reducing costs for both the wafer plating and subsequent CMP processes. It would also facilitate plating in the plated-side-up configuration that would lead to further improvements in the uniformity of the copper deposit and enable plating of finer IC features.
A low-profile cathode assembly could also provide similar advantages for other wafer plating processes. For example, solder bumps for flip chip attachment are often fabricated by electroplating tin-lead solder on wafer pads exposed through a photoresist mask. The pads are electrically interconnected by a metallic seed layer (often gold but other metals are used), which is subsequently removed from non-pad areas of the wafer by wet chemical etching. Typically, the whole wafer is immersed in the plating tank and electrical contact to the seed layer on the plated side is established via spring-loaded, plastic-shielded wires at a few points (usually three). Overplating of pads near the wafer edge is suppressed by use of plastic shielding in the plating solution. It is important that approximately the same amount of solder be plated on all pads within a given IC chip so that the solder balls are sufficiently uniform in height to be coplanar with the flip chip attachment pads on the substrate. As the trend toward IC chip miniaturization continues and solder balls decrease in volume, the requirement for solder plating uniformity across the wafer is becoming more stringent. Even if the coplanarity requirement within individual chips is met, too much solder in the balls can cause bridging that shorts the device. On the other hand, too little solder can result in structurally unsound solder joints because of inadequate underfill in the narrow space available, diminished distance over which stresses caused by thermal expansion mismatches can be relieved, and/or solder joint embrittlement induced by excessive volume fraction of gold contamination from seed or barrier layers. Consequently, there is an increasing need for a cathode assembly enabling pads on wafers to be plated with equivalent amounts of solder.
Our invention is a cathode assembly for semiconductor wafer plating that employs an electrically insulating coating on a ring-shaped electrically-conductive structural support to provide an improved seal to the seed layer near the perimeter of the plated side of the wafer. The coating also electrically insulates the conductive support structure and prevents electroplating on areas exposed to the plating solution. Consequently, a protective plastic housing is not required and the conductive structure can be utilized in making electrical contact to the wafer. The conductive material is preferably metallic and the insulating coating is preferably comprised of a polymer material. By using a very thin polymer coating and a strong metal for the structural ring material, the height to which the cathode assembly extends above the wafer surface can be made very small compared to that required for conventional gasket or o-ring seals. The latter are themselves relatively thick and are typically supported by a thick plastic structure. It is also preferable that the insulating coating be sufficiently adherent to maintain its position on the support structure during wafer changes.
A low-profile cathode assembly constructed according to this invention and having a beveled or sloping inner edge offers minimal interference to electrolyte flow across the wafer surface. This eliminates the need for complicated baffling and permits the relatively uniform solution flow needed for uniform copper deposition, which can be provided by agitation systems based on cathode rotation and/or pumped solution flow. In addition, since a protective plastic housing is not needed, the overall diameter of the cathode assembly, and consequently the inner diameter of the plating tank, can be reduced to minimize overplating of the wafer perimeter. A relatively simple plastic shield, which need not be attached to the cathode assembly, can then be used to provide an even more uniform current distribution. This low-profile cathode assembly is also amenable to use in the plated-side-up configuration, which facilitates removal of air bubbles from fine trenches and vias in the wafer. The plated-side-up configuration also provides more flexibility in designing the plating tank so that its inner diameter matches the exposed wafer diameter to avoid the problem of overplating the wafer perimeter.
In a preferred embodiment, the seal to the wafer surface is provided by a polymer-coated metal ring having a sawtooth cross section so that force applied to compress the seal is concentrated along concentric circular ridges so as to provide maximum resistance to solution leakage past the seal. Use of multiple ridges provides redundancy, improving the seal effectiveness. For some applications, a single ridge or a flat seal may provide satisfactory results. Stainless steel is a preferred metal for fabricating the cathode assembly since it has the high strength needed for fabrication of stiff structures of minimum cross section. It also resists corrosion that could result from plating bath or rinse water contact, either directly or via pores in the polymer coating.
A preferred polymer coating material is polytetrafluoroethylene (PTFE), which is relatively soft and highly hydrophobic so that it tends to flatten in contact with the wafer surface and repel plating solution to form a very effective seal. It also provides outstanding protection and insulation to the metal since it is chemically inert and repels solution from pores in the coating due to its hydrophobicity. Very thin PTFE coatings applied by thermal spraying are highly adherent to stainless steel (and other metals). For improved seal performance and to suppress electroplating through pinholes, the PTFE coating can be thickened by multiple applications.